Alternating current telegraph system



L. C. ROBERTS ALTERNATING CURRENT TELEGRAPH SYSTEM Jan. 3L 1950 12Sheets-Sheet 1 Filed Dec. 50, 1942 lllll ATTORNEV Jan. 3E, 19,50 l.. c.ROBERTS 2,495,773

ALTERNATING CURRENT TELEGRAPH SYSTEM Filed Deo. 30, 1942 12 Sheets-Sheet2 Q g A u CORD CCT /Nl/EA/TOR L C. ROBER 7'5 12 Sheets-Sheet 3 il Rm Am5 A L NB ZC I.. P Wm T h ||n| m Wn e u H H QMN\Q3D` Ly H m m ma v u n.Y

. Jan, 3L 950 1 c. ROBERTS ALTERNATING CURRENT TELEGRAPH SYSTEM Filednec. so, 1942 .FL RM WWIYBM Y F. v N7- Jan. 31, R950 L. c. ROBERTS2,495,773

ALTERNATING CURRENT TELEGRAPH SYSTEM Filed Deo. 30, 1942 12Sheens-ShfaecI 4 Jans 331, 1950 L. C. ROBERTS ALTERNATING CURRENTTELEGRAPH SYSTEM Filed Dec. 30, 1942 l2 Sheets-Sheet 5 ,Jann 3E, L, C,ROB-ERTS ALTERNATING CURRENT TELEGRAPH SYSTEM Filed Dec. 30, 1942 12Shees--Sheefl 6 'TOLL L//ve TERM/Anon dwf POL A R/ZE D /NVE/VTOR.CROBERTS Jan. 3,1, 1950 L. c. ROBERTS ALTERNATING CURRENT TELEGRAPHSYSTEM 12 Sheets-Sheet 7 Filed Deo. 50, 1942 A/A/l/fj/vof? L. C. ROBERTSATTO/Q [Y Jan. 31, 150 l.. c. ROBERTS 2,495,773

ALTERNATING CURRENT TELEGRAPH SYSTEM Filed Dec. 30, 1942 y 12Sheets-Shea?l 8 Jan. 3L i950 L. c RoBERTs ALTERNATING CURRENT TELEGRAPHSYSTEM 12 Sheets-Sheet 9 Filed DBC. 30, 1942 SDSU QQQU /NVENTOR L CROBERTS BV @c ATTOR V Jan.31,195o l L. c. ROBERTS l 2,495,773

ALTERNATING CURRENT TELEGRAPH SYSTEM Filed Dec. 30, 1942 12 Sheets-Sheet10 POLAR/IED POLAR/Z50 /NVE/VTOR L c. ROBERTS Jan. 31, 1950 L. c.ROBERTS ALTERNATING' CURRENT TELEGRAPH SYSTEM 12 Sheets-Sheet 1l FiledDec. 30, 1942 Lv I /NVE/VTR L c. ROBERTS Jan. 31, 1950 L. c. ROBERTSALTERNATING CURRENT TELEGRAPH SYSTEM 12 Sheets-Sheet 12 Filed Dec. 30,1942 /Nl/E/von BV .C. ROBERTS ATTO/QN V Patented Jan. 3l, 1950ALTERNATING CURRENT TELEGRAPH SYSTEM Leland C. Roberts, Towaco, N. J.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application December 30, 1942, Serial No.470,575

13 Claims.

This invention relates to printing telegraph apparatus and systems, andparticularly to a teletypewriter exchange system.

An object of the invention is to employ alternating current for thetransmission of signals between subscribers stations and central olcestations and also between subscribers stations interconnected throughcentral oiilce stations.

Another object of the invention is to interconnect through a centralofce switching station, subscribers stations which transmit and receivealternating current signals and subscribers stations which transmit andreceive direct current signals and to provide for conversion of directcurrent signals to -alternating current signals and alternating currentsignals to direct current signals in the central oce station equipment.

Another object of the invention is to interconnect subscribers stationsequipped for the reception and transmission of alternating currentsignals over long toll lines which transmit direct current signals withprovision for converting from alternating current to direct currentsignals 'and from direct current signals to alternating current signalsin central oiiice equipments at both ends of the long toll line.

Another object of the invention is to interconnect subscribers stationsequipped to transmit and receive alternating current signals over tolllines arranged to carry alternating current signals.

Another object of the invention is to provide a regenerative repeaterfor regenerating attenuated or otherwise deteriorated alternatingcurrent signals.

The invention features signal level compensation apparatus associatedindividually with intertoll trunks whereby a substantial saving inapparatus to be installed and maintained is accomplished as contrastedwith the provision of signal level compensation apparatus associatedwith each subscribers loop, since the number of trunks in a system isusually a small fraction of the number of subscribers loops servedthereby.

The invention also features an arrangement whereby break and othersupervisory signals are received and identified in a simple manner.

In accordance with the invention the subscribers stations areindividually provided with a source of alternating current, such as anelectron tube oscillator circuit, which generates alternating current atthe signaling frequency. The application of trains of alternatingcurrent waves to the subscribers line is controlled by transmittingcontacts, and transmitted alternating current as well as received-alternating current is amplied by a local amplifier and is rectifiedfor the purpose of operating a printing telegraph receiving recorderwhich records received messages, and in the case of locally generatedsignals, produces a home record of those signals. The subscribersstation operates upon a basis of alternating current olf for markingsignals and on for spacing signals, this arrangement providing foralternating current oi at the subscribers station while signals arebeing received, so that the receiving amplier is responsive to receivedtrains of altern-ating current waves without interference from locallygenerated alternating current, whereas it would not respond toreceivedtrains of alternating current waves separated by no-currentintervals if the subscribers transmitter, in the idle condition, shouldbe applying steady alternating current to the receiving amplifier.

At the central ofce station, cord circuits arranged to carry alternatingcurrent signals between two subscribers line circuits or between asubscribers line circuit and a trunk circuit are individually associableby means of keys with an operators set which is similar to a subscriberstransmitting and receiving set, in that it generates alternating currentfor signaling, amplies and recties received signals, and also ampliesand recties locally generated signals for the operators home record. Thecord circuit has supervisory apparatus at both ends for registeringsupervisory signals such as recall and disconnect signals.

Connections between subscribers served by different central officestations are established over intertoll trunk circuits extending betweenthe two central oiice stations. At each end the receiving sides of thetrunk circuit are provided with receiving amplifiers which haveautomatic gain control features. Since the gain of such ampliers is atmaximum when no signal is being received and such ampliers will then bemost sensitive to noise disturbances and also to the beginnings ofincoming signals, itis preferable to have the lines interconnecting thecentral office stations energized by alternating current for the idle ormarking condition in order to hold the gain of the receiving amplifiers-to norm-al level. Accordingly, at each end of the trunk in thetransmitting side there is a signal conversion circuit for convertingcurrent-on spacing signals received from a' cord circuit to current-olfspacing signals and, conversely, for converting currentoi markingsignals received from a cord circuit to current-on marking signals. Thetrunk circuits have supervisory apparatus at each end for registeringsupervisory signals, such as toll ringing, recall and disconnectsignals.

The invention recognizes the desirability of being able to transmitsignals originating at and destined for stations equipped to transmitand receive alternating current Signals over toll line circuits whichare arranged to accommodate only direct current signals and areaccordingly equipped with direct current repeater relays. For theaccomplishment of this the invention includes an intertoll trunk circuithaving signal conversion systems at each end for converting alternatingcurrent signals entering the trunk circuit from a cord circuit intodirect current signals and for converting direct current signalsreceived over the trunk circuit into alternating current signals to betransmitted into a cord circuit. This alternating current-direct currentconverting intertoll trunk circuit also has supervisory apparatus ateach end for registering supervisory signals.

When alternating current trains of waves are transmitted over long tolllines the signals may become impaired such as by tapering oif of theamplitude of the beginnings or ends of the trains of waves, so that theduration of the portions of the trains of waves at effective amplitudemay be materially reduced. For the correction of this condition theinvention includes a regenerative repeater for alternating currentsignals which produces trains of alternating current at full amplitudefor each received train of alternating current and which has timingmeans for allotting a full impulse interval to each produced train oralternating current independently of the duration of the portion of thereceived train which is of eiective amplitude.

In order to impart further flexibility to the system, particularly incontemplation of the possibility that a central oiice station may servesome subscribers stations that are equipped to transmit and receivealternating current signals and may serve other stations that areequipped to transmit and receive direct current signals, the

invention contemplates a cord circuit one end of which is adapted to beconnected to a subscribers station equipped to transmit and receivealternating current signals and the other end of which is adapted to beconnected to a subscribers station equipped to transmit and receivedirect current signals. The cord circuit includes means for convertingalternating current signals to direct current signals and for convertingdirect current signals to alternating current signals and has at eachend supervisory apparatus for registering supervisory signalsoriginating at the respective alternating current subscribers station ordirect current subscribers station. The operators set cooperates withthe alternating current portion of the cord circuit so that it alsogenerates and receives alternating current signals and such signals whentransmitted to the subscribers station equipped to transmit and receivedirect current signals are converted to such signals.

For a complete understanding of the invention, reference may be had tothe following detailed description to be interpreted in the light of theaccompanying drawings wherein:

Fig. 1 is a diagrammatic View showing subscribers stations equipped totransmit and receive alternating current signals, and their linetermination circuits at a central office station, one of the subscribersstations and associated line termination circuit being shown in fullschematic form and the other being shown symbolically;

Il i) Fig. 1A is a view in block diagram iorm representing a subscribersstation and subscribers line circuit which may be identical with thestation and line circuit shown in Fig. 1;

Fig. 2 is a diagrammatic view showing central ofiice station cordcircuits for cooperating with the subscribers line termination circuitsand for extending connections to other subscribers loops or to trunkcircuits, one of the cord circuits being shown in full schematic formand the other being represented symbolically;

Fig. 3 is a diagrammatic view showing an operators set and associatedsupervisory apparatus;

Figs. 4 and 5 when placed side by side with Fig. 5 at the right of Fig.4 form a diagrammatic View showing an intertoll trunk and relay circuitfor extending telegraph circuits over toll lines at the remote ends ofwhich is a similar intertoll trunk and relay circuit;

Fig. 6 in combination with Fig. 4 forms a diagrammatic view showing anintertoll trunk and reiay circuit with a conversion feature forextending a connection from a subscribers station equipped to transmitand receive alternating current signals over a toll line equipped tocarry direct current signals, there being at the remote end of the tollline a similar intertoll trunk circuit;

Fig. '7 is a diagrammatic view showing a connector circuit forconnecting a regenerative repeater between two communication circuitclements such as cord circuits;

Fig. 8 is a diagrammatic View showing a regenerative repeater forregenerating alternating current signals;

Fig. 9 is a diagrammatic view showing cord circuits for establishingconnections to the line termination circuits of toll subscribers, one ofthe cord circuits being shown in full schematic form and another beingshown in symbolic form;

Fig. 10 is a diagrammatic View showing the line termination circuit fora toll subscriber station equipped to accommodate direct currentsignals;

Fig. 11 is a diagrammatic View showing a toll subscribers stationcircuit;

Figs. 12 to 16 are diagrammatic views showing how the figureshereinbefore identified may be combined to show several systems throughwhich subscribers stations may be interconnected for communication, andas such systems involve duplication of apparatus at opposite endsthereof and in certain instances at intermediate points, some of therepresentations of the iigures have been shown in dotted outline toindicate that if duplicate drawings had been provided 4in suicientnurnber to enable any one of the systems to be laid out in its entirety,those figures indicated in dotted outline would be in reverse or mirrorimage form in order to bear proper relationship to the figures precedingthem;

More specifically, Fig. 12 represents a system for interconnecting twolocal subscribers served by a single central oice station and bothequipped for the transmission and reception of alternating currentsignals;

Fig. 13 represents a system for interconnecting two subscribers servedby individual central oflice stations, connection being established overintertoll trunk circuits and all signal transmission being upon analternating current basis;

Fig. 14 represents a system for interconnecting two subscribers Servedby separate centra-l oice stations, the 4central oilice stations being-connected :over a long toll line adapted to trans- :mit direct currentsignals, so that signal con- Version between alternating and directcurrent occurs in both central oiiice stations;

Fig. represents a system -for interconnecting two subscribers stations,the geographical separation between which is so great that the twocentral oflices which directly serve the subscribers stations becomeinterconnected lover intertoll trunks through an intermediate centraloffice station where a regenerative repeater is connected 4between theintertoll trunks. The signal transmission is on an alternating currentbasis throughout the system; and

Fig. 16 represents a system for interconnecting two subscribers servedby one central oiiice station, one of the stations being a localsubscriber equipped to transmit and receive alternating current signalsand the other subscriber fbeing a toll subscriber equipped to transmitand receive direct current signals so that a signal conversion takesplace in the central oflice station.

Local subscribers station and Zine circuit The system represented byFig. 12 will be first described and, accordingly, reference is had t0Fig. 1 in which a local subscribers station circuit I0| is connected tosubscribers line circuit |02 by line conductors |03 and |04. Thesubscribers line circuit |02 is individual to the subscribers stationI0| and is located at the central oiiice station.

The line conductors |03 and |04 terminate at the station I 0I in a loopwhich includes one winding of a transformer |06 and a key |01 `whichwhen closed short-circuits a ringer circuit |08 and conditions the linecircuit for signal transmission between the subscribers station and thecentral office station, and when opened, adds the ringer |08 to the loopcircuit in series with the right-hand winding of transformer |06. Theleft-hand winding of transformer |06 is connected to the right-handwinding of a transformer |09 through resistors and ||2 on one side andthrough resistor I|3 on the other side. A resistor I 4 is connected inshunt with the leithand winding of transformer |06 at the righthand endof resistors I|| and |I3 and a resistor I I6 is connected in shunt withresistor ||4 at the left-hand end of resistors I|I and ||3.

The left-hand winding of transformer |00 lhas one terminal connected toground and the Iother termin-al connected through high resistance gridleak I I'i shunted by condenser ||8 to the grid of an electron dischargetube IIB, the cathode of which is connected through a biasing resistorto ground. Tube H9 serves as an amplifier of trains of alternatingcurrent which represent code impulses of permutation code signals, andthe anode of tube I|9 is connected through the primary lwinding oftransformer |2| to anode battery |22. The secondary of transformer |2|is center tapped and both halves of the winding are connected inparallel with one winding of a biased polar relay 25, the lower orbiasing winding of which moves the armature to the marking contact whenno current flows in the upper wind-` ing which, when energized,overcomes the biasing winding and moves the armature to spacing. In themarking condition the relay completes the circuit of the selector magnet|23 of .a printing telegraph receiving recorder which may be of the typeshown in Patent 1,904,164, granted April 18, 1933, to S. Morton et al.The disclosure of the Morton patent is incorporated herein 'by referenceas part of the present specification, and it is to be understood thatthe selector magnet |23 symbolizes a .receiving printing telegraphrecorder. `Rectliiers |24 are so associated lwith the two halves of thesecondary winding of transformer |2| that current through the upperwinding of relay |25 will always be in the same direction whethercurrent in the anode circuit of tube i I9 is rising 4or falling.

'Subscribers station |0I is provided with a `generator of' alternatingcurrent in the form of an electron tube oscillator I 3| The anodecircuit of oscillator I3! includes the .primary winding of a transformer|32 through which anode `cu-rrent flows from anode battery |33. 'I'hesecondary winding of trans-former |32 is connected to the primarywinding of a transformer |34 Athrough a resistor |36 on one side anddirectly on the other side and the second-ary of transformer |34 isconnected in parallel with resistor I6 and thus in parallel with thelefthand winding of transformer |06 and with the primary of transformer|09, Oscillator I3I generates oscillatory signaling current which istransferred through transformer |32, transformer |34 :and transformer|06 into line conductors |03 and |04 for transmission to the centralolce station and also through transformer |09, amplifier tube I I9,transformer |2| and rectii'lers |24 to selector magnet |23 for theproduction of a home record corresponding to signals generated at thesubscribers station 0 The selector magnet |23 is responsive either torectified signals transferred through transformer |06 from lineconductors |03 and |04 or to 'rectified signals transferred throughtransformer |34 from oscillator |3|.

Transmitting contacts |4| which may be controlled by a keyboardmechanism as shown in the Morton et al patent are contained in anormally closed circuit which includes break key |42, battery |43,winding of relay |44, and winding of relay |45. When break key |42 isclosed and one of the transmitting contacts |4| is closed, which is themarking condition, relays |44 and "|46 are energized to hold theirarmatures on the marking contacts. When relay |44 is energized, itshort-circuits the `resistor 2 through its armature and marking contactand when the relay isv deenergized, it opens the short-circuit acrossresistor I I2 and completes no circuit on its spacing contact. When therelay |46 is energized, it short-circuits the secondary winding oftransformer |32 through its armature and marking contact and when relay|40 is deenergized, it short-circuits-the resistor |30 through itsarmature' and spacing contact. Relays |44 and |46 preferably are biasedpolar relays whereby the armatures move to the spacing position whenbreak key |42 or all of the transmitting contacts 4| are open and whenbreak key |42 and any of `thetransmitting contacts are closed, thespacing bias is overcome and the armatures move to the marking contacts.

Since the output of oscillator I3! is short-circuited through thearmature and marking contact of relay I 45 when transmitting contacts 4|are in the rest or idle condition, which is a marking condition, it willbe apparent that no oscillatory .short-circuit across the secondary oftransformer |33, and goes to its spacing contact, thus shortcircuitingresistor |36, so that alternating current at full amplitude is appliedto the primary winding of transformer |34` and corresponding current isinduced in the secondary winding of that transformer. In the transitinterval of relay |46, spacing current flows at less than full amplitudebecause resistor |35 has not yet been short-circuited. This arrangementfor keying the oscillator is different from another keying circuit whichwill be described later, in that the spacing oscillatory current beginsto flow in the primary winding of transformer |34, although at less thanfull amplitude, the instant that the armature of keying relay |46 leavesits marking contact, whereas in the arrangement to be described later,the spacing oscillatory current is not applied to the output circuit orto the receiving ampliiier circuit until after the keying relay reachesits spacing contact. The latter arrangement may introduce bias in thesignal due to the travel time of the keying relay, whereas in the keyingarrangement shown in Fig. 1 such bias is avoided.

The purpose of relay |44 is to compensate for signal level differencesbetween locally generated and received signals. It will be apparent thatreceived signals will be of lesser amplitude than locally generatedsignals due to the attenuation of received signals during transmission.It will also be apparent that the receiving amplifier circuit must besensitive to received signals. Signals generated at the station shown inFig. 1 must be of suicient amplitude to operate the receiving amplifierat the central omce station or at a remote station and since suchsignals are also applied to the receiving amplifier locally for homerecord purposes, their amplitude may be such as to overload theamplifier. The signal level compensation is achieved by short-circuitingthe resistor ||2 when relay |44 is in the marking condition, which isthe condition in which it should be when the subscribers station isreceiving signals, it being remembered that the local oscillator isapplying no tone to the receiving amplier circuit or to the linecircuit, so that spacing trains of oscillator current will be applied tothe primary of transformer |06 as received over the line conductorswithout attenuation by resistor ||2. However, when the station shown inFig. 1 is transmitting, each time relay |46 operates to spacing, totransmit a spacing train of oscillatory current, relay |44 will move tospacing and. will open the short-circuit across resistor H2, thusproviding for reduction in the amplitude of the oscillatory currentapplied to the primary of transformer |09.

The grid resistance-condenser combination associated with the receivingampliiier is for the purpose cf rendering the amplifier unresponsive toecho current. The outgoing signals, although impressed on the ampliiierthrough resistor H2, are strong enough to cause the grid to becomepositive and, therefore, charge the condenser. Until the condenserbecomes discharged, the amplier tube I5, which is of the variable mutype, is less sensitive to incoming signals, and thus will not respondto echo currents.

At the central oice station the line conductors |03 and ||E4 terminatein subscribers line circuit |02 which comprises answering and callingjacks and call indicators. Specifically, line conductor |53 terminatesat the tip spring of calling jack I5| and conductor |04 terminates atthe ring spring of this jack. The break contacts which are normallyengaged by the tip and ring contact springs of calling jack |51 when thejack is idle are connected to the tip and ring contact springs ofanswering jack |52. Each of the jacks |5| and |52 has a grounded makeContact through which ground will be connected to the sleeve of the jackwhen a plug is inserted into the jack and the sleeves are connectedtogether so that both will be grounded upon the insertion of the pluginto either of the jacks. It will be understood that multiple answeringand calling jacks may be provided if desired so that more than oneoperator may have access to the subscribers line circuit and that thesleeves of all of the jacks will be grounded upon the insertion of aplug into any one of them.

The break contact which is normally engaged by the tip spring of theanswering jack |52 is connected to ground and the break contact which isnormally engaged by the ring spring of jack |52 is connected through thewinding of line lamp relay |53 to grounded battery. The front contact ofrelay |53 is connected to ground and the armature is connected throughline lamp |54 to grounded battery.

When the subscribers station shown in Fig. 1 is in the idle condition,key itl which is a call, recall and disconnect key, is open and ringer|08 is connected in the line loop. Due to the presence of condensers inthe ringer circuit, direct current will not iiow and relay |53 will bedeenergized. When it is desired to set up a call from the subscribersstation shown in Fig. l, key iC-'i ie close This completes a circuitfrom ground through the break contact and tip contact spring of jack|52, break contact and tip Contact spring of jack 55|, line conductorright-hand Winding of transformer |06, closed key |97, line conductor|04, ring contact spring and break Contact of jack |5i, ring contactspring and break contact of jack |52 and winding of relay |53 togrounded battery. Relay |53 will be energized over this circuit and willattract its armature to complete the circuit for lamp |54 which will belighted to indicate to an operator at the central office station that acall is awaiting on answering jack |52.

Central ojce cord circuit, operators set and supervisory apparatus Theoperators cord circuit and associated apparatus is shown in Figs. 2 and3. At the left of Fig. 2 is an answering plug 2M which the operatorinserts into jack |52. By this operation the circuit of line lamp relay|53 is interrupted at both break contacts of `iack |52 and thesubscribers loop circuit is extended over the tip and ring of plug 20|through the swinger springs and break contacts of subscriber ringing key262 and through conductors 203 and 2M, condensers 26.6. and 201 to theswinger springs of the left-handed side of splitting key 208.

As a further result of the insertion or plug 20| into jack |52, groundis connected to the sleeve of the jack through the auxiliary makecontact of jack |52 and through the sleeve of plug 20|, lamp 269,conductor 2|! and winding of relay 2|2 to grounded battery. Over thiscircuit lamp 209 is lighted and relay 252 becomes energized. At theinnermost upper armature and. iront contact of relay 2|2, ground isconnected through the upper winding of a relay 2|3, conductor 2|4, upperbreak Contact and upper swinger spring of subscribers ringing key 232,through the tip circuit of plug 20| and jack |52, subscribers loop whichis closed at key |07, ring circuit oi' jack |52 and plug 20|, lowerswinger spring and break contact of key 202, conductor 2|6 lower'winding of relay 2|3 and innermost lower armature and front contact ofrelay 2| 2 to grounded battery. Upon the energization of relay 2I3 acircuit is completed from ground through its lower armature andjrontcontact, conductor 2|?, winding of relay 2|8, conductor 2 9 and outerlower armature and front Contact of relay 2|2 to grounded battery. Relay2 l 8 which has a slow-release characteristic becomes energized and atits armature and front contact prepares an energizing circuit for arelay 22, which at this time is open at the back contact and lowerarmature of relay 2|3 since relay 2 i 3 has just become energizedthrough the lower armature and front contact of relay 2|3. At the outerupper armature of relay 2|2, a substitute energizing circuit for therelay is completed from grounded battery through the relay winding,front contact and outer upper armature, front contact and outer upperarmature of relay 2|3, conductor 222, back contact and upper armature ofa relay 223 and upper armature and back contact of relay 22| to ground,this path being in short-circulating relation to lamp 239 so that thelamp is extinguished.

The left-hand hall of the cord circuit shown in Fig. 2 is now fullyprepared to supervise action taken by the subscriber at the stationshown in Fig. l, and the operator at the central oliice station can nowcommunicate with the subscriber for the purpose of determining theidentity of the station called. For this purpose the break con tactsengaging the upper and lower swinger springs of both halves of splittingkey 208 to which the subscribers loop has been extended are connectedthrough conductors 228 and 229 and through condensers 226 and 22? to theswinger springs of an operators set connecting key 235, the makecontacts oi which are connected through conductors 232 and 233, 332 and333 to the left-hand winding of a transformer 334. The right-handwinding of transformer 334 is connected to an operators set 33!) whichmay be identical with the subscribers station apparatus shown in Fig. lwith the exception of the omission of a call key and ringer apparatus,but which has been shown in Fig. 3 as being provided with a differenttype of. keying apparatus, to which reference has been made previously.Since the receiving portion of the operators set is like that shown inFig. 1 it will not be described in detail.

As to the transmitting apparatus, the circuit of the transmittingcontacts and break key includes the windingI of a single relay 335 whichls preferably a biased polar relay operable to move the armature to thespacing contact when the circuit of the operating winding is open butholding its armature on the marking contact when 1e circuit of theoperating winding is closed.

The source of alternating signaling current 336, which has beenindicated conventionally for the sake of simplicity but which may -be anelectron tube oscillator as in the showingl in Fig. l, has one terminalconnected to one side of the right-hand winding of transformer 334 andthe other terminal connected to the spacin-g contact of keying relay335. The side of the right-hand winding of transformer 334 to whichalternating current source 336 is connected is also connected to oneterminal of the primary winding of a transformer 331, the secondary ofwhich serves as the input for the amplier tube 338. The armature ofrelay 335 is connected through re,- sistor 339 to the upper terminal ofthe rightfhand winding of transformer 334 and through resistor 34| tothe upper terminal of the primary winding of transformer 331. Themarking contact of relay 335 is connected to the right-hand end ofresistor 33|. When relay 335 is in the marking condition, the source ofalternating signaling current 333 is disconnected from transformers 334and 33'! so that no oscillatory current is being supplied to thosetransformers, which is the marking condition, as hereinbefore set forth.Also the armature and marking contact of relay 335 short-circuitsresistor 34| which it is desired te interpose ahead of the ampliercircuit only in the case of strong locally generated signals. When thearmature of relay 335 moves to the spacing contact, the source ofalternating current 336 is connected across the right-hand winding oftransformer 333 and across the primary windingcf transformer 331 for thetransmission of alternating current signals over conductors 332 and 333and for the local recording of signals corresponding thereto. In thiscase the resistor 33| is included between the source of alternatingcurrent signals and the amplifier tube 338 for reducing the amplitude ofthose signals. It is to be noted that following the opening of the breakkey or of the transmitting contacts of the operators set, the initiationof the spacing signal is delayed by the transit time of the armature ofrelay 335 until the armature reaches the spacing contact, which maysubject the signals to a bias. If it is found that such bias is present,the type of keying circuit shown in Fig. 1 may be employed in theoperator's set.

The operator connects the transmitting and receiving set 33|! overconductors 332, 333, 232 and 233 to the conductors 228 and 229 by movingthe swinger springs of key 23| into engagement with their make contacts.The operator may then manipulate the keyboard transmitter of set 330 toinquire of the Subscriber as to the `identity of the station with whicha connection is desired. Assuming that connection is desired to anotherlocal subscriber such as the subscribers station |6| in Fig. l,identical with station ||J| having subscrlbers line circuit |62identical with the subscribers line circuit |32, or such as asubscribers station indicated in the dotted outline of Fig. 1 at theright-hand side of Fig, 13, such connection may be completed directlythrough the right-hand portion of the `cord circuit shown in Fig. 2.This portion of the cord circuit is identical with the left-hand portionpreviously described, having subscriber ringing key 242 corresponding tokey 232, relay 233 corresponding to relay 2|2, relay 244 correspondingto relay 2|3, relay 233 corresponding to relay 2|8, relay 241corresponding to relay 22|, relay 248 corresponding to relay 223 andlamp 233 corresponding to lamp 239.

The operator tests the called subscribers line for a busy condition bytouching the tip of plug- 24| to the sleeve of the calling jack of thecalled subscribers line circuit. II" the called subscribers line isbusy, there will be a ground connection on the sleeve of the jack due tothe fact that a plug is already inserted in some other jack associatedin multiple with the subscriber's line circuit, and the groundconnection will be extended over the tip of plug 23 l, tip swingerspring and break contact of key 242, conductor 25|, inner upper armatureand back contact of relay 244, outer upper armature and back contact ofrelay 243, conductor 252 and busy lamp 253 to grounded battery. It is tobe noted that busy lamp 253 also serves the left-hand half of the cordcircuit by being also connected to the back contact associated with theouter upper armature of relay 2|2. If lamp 253 does not light, thisindicates that the called subscribers line is not busy and accordinglyplug 24| is inserted into the called subscribers calling jack.

The operation of the springs of the called subscribers calling jackplaces a ground on the sleeve of plug 24| which causes lamp 249 to belighted and relay 243 to be operated. Relay 243 seeks to complete theenergizing circuit for relay 244, but as in the case of relay 2 3, thecircuit of relay 244 can be completed only through the loop circuit atthe subscribers station which, in the case of a called subscriber, willbe open to direct current at the subscribers call key. Since relay 244will not be energized at this time, the substitute energizing circuitfor relay 243 from ground on the back contact of the upper armature ofrelay 241 will be interrupted at the outer upper armature of relay 244so that relay 243 will be energized only through lamp 249 and the lampwill remain lighted.

The operator rings the called subscriber by operating ringing key 242 toconnect ground to the tip of plug 24| and source 254 of alternatingcurrent, usually having a frequency of the order of 20 cycles per secondfor ringing current, to the ring of plug 24|. The ringing current willoperate the ringer at the called subscribers station which is includedin the subscribers loop because the call key is open. Presumably, theoperator will not ring continuously but only intermittently, restoringkey 242 to normal condition after each ringing interval, awaitingevidence that the called subscriber answers. The call is answered byclosure of subscribers call key which short-circuits the ringer andcompletes through the loop the energizing circuit for the relay 244.

The relay 244 upon being operated, energizes relay 246 which preparesthe energizing circuit for relay 241, completes the substituteenergizing circuit for the relay 243 in short-circuiting relation to thelamp 249 which becomes extinguished, and is an indication to theoperator that the called subscriber has answered.

The called subscriber having answered, the operator may then transmitsignals from the transmitter of the operators set to instruct thecalling subscriber to proceed with message transmission. This may beaccomplished without transmission of the instructions to the calledsubscriber by operating the right-hand half of the splitting key 298,the eiect ofy which s to disconnect the tip and ring conductors of therighthand portion of the cord circuit from the conductors 228 and 229and also from the tip and ring conductors of the left-hand portion ofthe cord circuit and to interconnect the tip and ring conductors of theright-hand portion of the cord circuit through condenser 256 andresistor 251 in series. Having instructed the calling subscriber toproceed, the operator restores the right-hand half of the splitting keyto normal condition to reconnect the right-hand portion of the cordcircuit to the left-hand portion and also restores the key 23| to normalcondition to disassociate the operators set from the cord circuitthrough which the calling and called subscribers are interconnected,thus freeing the operators set for answering another call.

Subscriber break signals Either subscriber may send a break signalmerely by opening his break key such as the key |42 in Fig. 1. This willcause a continuous spacing signal to be transmitted during the intervalthat the break key is held open, and the break mechanism with which thereceiving printers at the interconnected stations may be provided asdisclosed in the Morton et al patent, will respond to the break signal.No supervisory operation occurs in the cord circuit as a result ofsubscriber break signals.

Subscriber recall signals Either subscriber may recall the operator atthe central oice station by momentarily opening and then reclosing hiscall key. Assuming that the subscriber at the station shown in Fig. lmomentarily opens the call key il to recall the operator, the circuitthrough the upper and lower windings of relay 2|3 will be interruptedand this relay will release its armatures. The ground connection on thelower armature of relay 2|3 will be transferred from the Winding ofrelay 2|8 to its armature. Relay 2|8 has a slow-release characteristicand will not release if the relay 2|3 remains released only momentarily.Accordingly, relay 22| will become energized from ground on the lowerarmature of relay 2 i3 through the back contact, armature and frontcontact of relay 2|8, winding of relay 22|, conductor 2I9, inner lowerarmature and front contact of relay 2|2 to grounded battery. Althoughthe ground connection for relay 2|2 in short-circuiting relation to lamp209 is interrupted when the outer upper armature of relay 2|3 isreleased, relay 2|2 remains energized from ground on the sleeve of jack|52 through lamp 209, so that the lamp becomes lighted. At its lowerarmature and iront contact, relay 22| establishes its own holdingcircuit to ground through the auxiliary break contacts of key 23| toground. At the upper armature of relay 22|, direct ground connection onthe back contact is replaced by a ground connection supplied through aninterrupter device 258 which supplies ground connection intermittentlyat a suitable rate, such as one impulse per second. The intermittentground connection thus provided, short-circuits lamp 299 and causes thelamp to ash, which is an indication to the operator to cut in on thecord circuit by operating key 23| to the oir-normal condition. When key23| is operated to enable the operator to check by telegraphiccommunication as to the reason for the recall, the auxiliary breakcontacts of key 23| are opened thus interrupting the holding circuit forrelay 22| which becomes deenergized, restoring steady ground for holdingrelay 2|2 in short-circuiting relation to lamp 209 so that the lamp isextinguished.

Subscriber disconnect signals A subscriber disconnects at the end ofmessage transmission by opening his call key. Assuming that the key |01in Fig. l is opened and remains open, the operation in the cord circuitwill be the same as that involved in the recall signal, namely, theenergization of relay 22| and establishment of its locking circuit andsubstitution of intermittent ground connection for steady groundconnection to the inner upper armature of relay 2|3. However, since thekey |81 is not reclosed, relay 2|3 remains deenergzed and relay 2 |8releases, which has no effect on the relay 22| since its locking circuithas already been established. However, the intermittent groundconnection is not extended to the winding of relay 2| 2 inshort-circuiting relation to the circuit of lamp 209 because the outerupper armature of relay 2| 3 remains out of engagement with its frontcontact since relay 2|3 is released. Accordingly, the interruptor 26|will have no effect on lamp 209 and it will burn steadily in the holdingcircuit of relay 2|2. The operator interprets the steadily illuminatedlamp 209 as a disconnect signal and withdraws plug 20| from jack |52.The operator also withdraws plug 24| from the calling jack of the otherlocal subscriber if lamp 249 has become illuminated steadily to indicatethat that subscriber has opened his call key. The withdrawal of theplugs 20| and 24| from the jacks which they had been engaging interruptsthe circuit of lamps 209 and 249 and of relays 2|2 and 243 in seriestherewith respectively. Relays 2|? and 243 disconnect battery fromrelays 22| and 241 respectively, thus releasing those relays and thecord circuit is restored to normal condition.

Intertoll trunk and relay circuit The next system to be described isthat delineated in Fig. 13 involving interconnection of a localsubscriber of one central oice station with a local subscriber ofanother central ofiice station over an intertoll trunk circuit extendingbetween the two central ofce stations. Each of the two subscribersstations to be interconnected may be of the type shown in Fig. 1 and theapparatus at each central ofce station including the cord circuitoperators set and associated apparatus may be of the type shown in Figs.2 and 3. The intertoll trunk and relay circuit is shown in Figs. 4 and5. The trunk circuit termination is shown in detail for one end of thetrunk circuit, such apparatus being located at one of the central officestations and the duplication of this apparatus at the remote centraloiice station is indicated diagrammatically by the rectangles. jack andlamps at the right of Fig. 5.

Assume that the operator has answered a call from the subscribersstation shown in Fig. 1 by the insertion of plug 20| into jack |52, hasas- 4 certained the identity of the called subscriber, and inserts plug24| into jack 40| of the intertoll trunk circuit. The tip contact springof jack 40| is connected to conductor 402 which extends to the upperterminal of inductance 403 and to the left-hand terminal of condenser404. The ring contact spring of jack 40| is connected to conductor 406which extends through the armature and back contact of slow-releaserelay 401 to the lower terminal of inductance 403 and to the left-handterminal of condenser 408. With the insertion of the plug into jack 40|,auxiliary make contacts of the jack are closed whereby a circuit iscompleted from grounded battery 409 through the closed make contacts,conductor 4|| and winding of relay 4|2 to ground. Relay 4I2 becomesenergized and through its left-hand armature and front contact extends aground connection over conductor 4|0 to the sleeve of jack 40|, fromwhich the connection is extended through the sleeve of plug 24|, lamp249, and winding of relay 243 to grounded battery. Relay 243 causes theupper and lower windings of relay 244 to become energized, the circuitbeing extended through the tip and ring contact springs of jack 40| andover conductors 402 and 406 bridged by inductance winding 403 to form aclosed direct current loop. Lamp 249, which became operated in serieswith the winding of relay 243 becomesextinguished due to theestablishment of a. short-circuiting ground connection from the upperback contact of relay 241 through the upper armature and back contact ofrelay 248, the outer upper armature and front contact of relay 244 randthe outer upper armature and front contact of relay 243 over whichcircuit the relay 243 is held energized. Relay 244 also causes relay 246to become energized, thus preparing an energizing circuit for relay 241.These operations are identical with those which occurred when theoperator answered a call from the subscribers station shown in Fig. 1 bymeans of plug 20|.

Besides establishing a circuit for operating relay 243 in the right-handportion of the cord circuit shown in Fig. 2, the relay 4|2 establishes acircuit from battery on its right-hand armature through the frontcontact and through the righthand winding of relay 4|3. Relay 4| 3becomes energized and through its armature and front contact connectspower to busy lamp 4|4 which lights. All jacks which are connected inmultiple with jack 40| have busy lamps associated therewith and suchlamps are connected in multiple with lamp 4|4 so that the busy conditionis indicated at all multiple jack terminations of the left-hand end ofthe trunk circuit shown in Figs. 4 and 5. Thus it is not necessary foran operator to test for a busy condition of the intertoll trunk since abusy condition of a trunk is positively indicated by lighted lamps atall appearances of its jack terminals.

Having thus conditioned the near end of the intertoll trunk circuit foroperation, the operator rings the distant end of the intertoll trunk byopening the left-hand side of splitting key 208 to keep the ringingsignal out of the local sub-- scribers side of the cord circuit and byclosing the toll ringing key 259 for an interval of approximately two tofive seconds. The ring toll key connects a source of alternating current3I4 at the frequency employed for telegraphic communication in thesystem shown, through conductors 31| and 312, conductors 21| and 212,closed contacts of ring toll key 259, condensers 226 and 221 and theclosed contacts of the righthand portion of the splitting key andthrough the tip and ring of plug 24| and tip and ring contacts oi' jack40| to conductors 402 and 406. Thus a spacing signal having a durationof two to five seconds is transmitted into the intertoll trunk circuit.

The right-hand terminal of each of the condensers 404 and 408 isconnected through conductors 405 and 4|5 to a hybrid coil 5|6 and thespacing impulse trains of alternating current ilow through thecondensers 404 and 408 to this coil. The condensers 404 and 408 block oifrom the hybrid coil direct current which iiows in conductors 402 and406 for energizing the relay 244, and the inductance 403 provides adirect current path for this current in addition to serving as a ilterin combination with the condensers 404 and 408. For reasons which willbe described hereinafter, it is desirable that in the trunk circuit,marking signals shall be represented by a Aiiow of alternating currentand spacing signals .shall be represented by a no current condition.

opposite of the condition in the Since this is thev local subscribersline and in the cord circuit, it is necessary to provide a signaltransposing circuit associated with the intertoll trunk circuit. Thistransposing circuit will now be described.

The spacing signals comprising trains-of alternating current inducealternating currents at the same frequencies in windings 511 and 518 ofthe hybrid coil. These coils are in series, and a condenser 519 shuntedby a resistor.521 has one terminal connected to the point ofinterconnection of the two windings 511 and 518. The other ends of thewindings 511 and 518 are connected through half-wave rectiflers 522 and523 to the other terminal of condenser 519. This circuit serves as afull-wave rectifierforrectifying the spacing trains of alternatingcurrent received over conductors 492 and 456 and for charging condenser519 with such rectified current. There is also connected acrosscondenser 519 andresistance 521 a circuit comprising a source ofAalternating current 525, which generates current at the signalingfrequency, condenser 521, resistor 528, primary winding of transformer529 and rectifier 531. When the full-wave rectiiier circuit, includingrectiflers 522 and 523, is charging condenser 519, the condenser blocksthe flow of current from alternating vcurrent source 521i through theprimary of transformer 529 so that no current flows in that winding ofthe transformer. When current ceases in conductors 402 and 405 which isthe marking condition, condenser 519 discharges and permits current toiiow through the primary winding of transformer 52S and rectifier 531 sothat a marking signal of alternating current ow is induced in thesecondary of transformer 529. Thus as a result of the toll ringingspacing signal lasting from two to five seconds, no current is generatedin the secondary winding of transformer 529. This winding of thetransformer is connected through band-pass filter 532 to toll lineconductors 533 and 534 which extend to the remote central ofiice stationwhere they enter the receiving circuit of an intertoll trunk and relaycircuit identical with that shown in Figs. 4 and 5. In the signaltransposing circuit, a battery 535 has one terminal connected throughresistor 531, primary winding of transformer 529 to one terminal ofrectifier 531 and the other terminal of the battery is connected throughresistor 521 to the other terminal of rectifier 531. This batterynormally biases the rectifier 531 so that it operates on the linearportion of its characteristic. Y

,Since only one end of the intertoll trunk circuit has been shown indetail, namely, in Figs. 4

and 5, it is necessary to consider the effect of received call and othersignals as if received from the remote station shown at the right ofFig. 5. in considering the receipt of the toll ringing signal inparticular, it will be necessary to con sider the relays 412 and 413,heretofore described as having been energized due to the insertion ofplug 241 into jack 40|, as being released. Signals are received over aseparate pair of receiving conductors designated by'the referencenumerals 541 and 542 and signals received over these conductors areimpressed upona bandpass filter 543, the output of which is connected tothe primary winding of transformer 544. The reason for providingband-pass lters 532 and and the lter 543 suppresses from the lgnirvnary543 is that the conductors 533 and 534 may carry "65 winding oftransformer 544 all but one message frequency or band of frequencies.

The secondary winding of transformer 544 is connected to the gridcircuit of an amplifier tube fifthe anode circuit of which is coupledthrough a resistance capacity coupling to a second stage amplifying tube541. The anode circuit oi tube 551 includes the primary winding of anoutput transformer 548, the secondary winding of which is center tappedand is connected in a full-wave rectifier circuit 555 similar to thatpreviously described comprising the hybrid coil windings 55.7 and 518,the rectiiiers 522 and 523 and condenser i9 shunted by resistor 521. Therectifier circuit 555 controls the flow of current from a source cioscillatory current 549 through the primary winding of a transformer 551having a 'nali-wave rcctier 552 in the same relation to source ofoscillatory current 549, the rectier circuit 1i and the primary windingof transformer' 551 that rectifier' 531 has to source 0f oscillatorycurrent 546, the full-wave rectifier circuit which includes condenser519 and the primary winding of transformer 529. Thus .marking intervalsof oscillatory current received over the line conductors 541 and 542 areamplified in the two stage amplifier and are transposed into markingsignals of no current condition in the secondary of transformer 551. Thesecondary ci transformer 551 is connected to the input of a band-passfilter 551i, the output of which connected into the hybrid coil 516whereby the transposed signals are impressed on conductors 452 and 455by which they are conducted to the cord circuit shown in Fig. 2.

Transformer 548 has a tertiary winding which is connected across theinput terminals of a full Wave bridge rectifier circuit 554, the outputclrcuit of which includes resistor 556, condenser 551 shunted acrossresistor 556, and a series of two condensers 558 and 55S shunted acrosscondenser 551, the condenser 55S having a resistor 551 in shunttherewith. From the point of interconnection of condensers 558 and 559,conductor 552 is connected back to the grid circuit of ampliiier tube556. The rectifier circuit 554 and associated apparatus serves as auautomatic gain control system rectifying a portieri of the output of thesecond stage amplifier tube 541 and applying back to the grid circuit,over conductor 552, a biasing voltage which supplements the nor-'nalbiasing voltage provided by the cathode ref 1 of tube 5&5 and whichvaries inversely as the signal level. When a weak signal is lmpressedupon the grid of tube 546, the voltage impressed across rectier circuit554 is less than when a strong signal is received and accordingly tube5156 is biased less negatively as a result of reception of a weak signalthan of a strong signal. Thus a substantially constant level of signalirnpressed on the secondary ol transformer 54B can be maintained.

The automatic gain control feature having been described, the reason forthe transposition of signals so that the trunk circuits shall carryoscillatory current for the marking condition and no current for thespacing condition may be explained. The idle condition of conductors 533and 534 will be thc marking condition v ith oscil latory current flowingsince, with no current flowing in conductors 552 and 456, whetherbecause of the fact that no plug is inserted into jack 481 or because ofan idle interval or lull in communication between two stations connectedover the trunk circuit, the signal rectifier in the transposi-

